This invention relates to sheet-handling apparatus, and more particularly to a device for holding down sheets which are stacked on a support.
In many types of sheet-handling apparatus, for example in printing apparatus or other types of equipment, sheets must be individually supplied to a working station from a stack of sheets. Stacked sheets, however, have a tendency to adhere to one another and in order to overcome this and to assume the feeding of only single sheets to the working station, it is known to "loosen up" the stack by blowing air between the stacked sheets. This does separate the sheets from one another; however, in the upper zone of the stack the sheets then start to "float" due to the air cushion between them and to shift transversely relative to each other. Because of this the edges of the vertically stacked sheets move out of vertical alignment and as soon as one or more of the sheets has moved laterally out of its proper position the apparatus will malfunction and shut down, because the working station can accept the sheets only in their proper positions.
Various arrangements have been proposed to overcome this problem.
Thus it has been suggested to provide two laterally adjacent vertical rods each carrying at its lower end a steel ball which rests under the influence of gravity on the uppermost sheet of the stack, in order to exert upon the stack a pressure which is intended to be constant and not influenced by fluctuations in the stack height. However, it has proved to be impossible to obtain and maintain an optimum pressure level in this manner.
A further proposal suggests a similar arrangement in which, however, the pressure-exerting elements are spring-loaded, i.e., are each biased by a spring against the upper sheet of the stack. The pressure exerted is adjustable by means of an adjusting mechanism for the spring. Here the pressure exerted on the sheet stack will evidently vary in dependence upon the fluctuating height of the stack; therefore, assuming that the desired pressure is the one which is exerted when the stack has its maximum height, then the pressure must be readjusted (i.e., increased) as the height of the stack decreases. This is cumbersome and never completely accurate or predictable, especially as it is almost impossible -- certainly within economically feasible expenditures -- to produce two springs having exactly identical spring characteristics or, if that is not the case, to separately readjust each spring so that it exerts exactly the same biasing force as the other spring of the arrangement.
A third proposal suggests vertically slidable rods which rest on the upper sheet of the stack and are provided with retainers into or onto which different weights can be placed. Adjustments in the contact pressure are effected by manually adding or removing respective ones of the weights. This has the disadvantage that the readjustments are time-consuming and that there are times when one rod is subjected to a greater weight than the other rod, e.g., when a weight has been removed from one rod but before the same amount of weight can be removed from the other rod.
In summary, therefore, it can be said that the prior proposals do not or not with adequate certainty assure that only single sheets are fed from a stack to a working stacking station, and that they are fed in the required orientation. Moreover, during brief machine shut-down incidents -- during which the air usually continues to be blown between the sheets of the stack -- these proposals cannot assure that the sheets do not shift laterally since the pressure force exerted upon the stacked sheets cannot be rapidly enough increased with the manual means provided according to these proposals.